Field of the Invention
The present invention relates to a gripper system for a machine that processes printing material sheets. The gripper system includes a gripper having a first clamping surface and a gripper pad which cooperates with the gripper and has a second clamping surface. One of the two clamping surfaces has a surface relief.
German published patent application DE 31 46 836 A1 describes a gripper system, the clamping surface of which has a surface relief formed from prismatic profile elements. Two situations can be assumed with regard to that clamping surface.
There is no problem in one situation in which the clamped printing material sheet completely covers the clamping surface. In this situation, the clamping force acts over the entire clamping surface as intended and, as a result, the surface pressure exerted by the clamping surface on the printing material sheet is not excessive. The surface relief or its prismatic profile elements only dig into the printing material sheet as deeply as is necessary in order to hold the printing material sheet securely in the gripper system.
In the other situation, which is indeed problematic, the clamping surface is only half covered by the sheet corner of the clamped printing material sheet in the direction of its sheet width. As a result, the clamping force, whose magnitude is unchanged with respect to the first situation, is now distributed exclusively in the partial region of the clamping surface that is covered by the printing material sheet and no longer over the entire clamping surface. This concentration of the clamping force in turn has the consequence that the surface pressure acting on the clamped printing material sheet is increased with regard to the first situation and, for example, is approximately doubled, and that the prismatic profile elements dig into the printing material sheet to an excessive depth. The increase in the surface pressure and the excessive digging in of the profile elements can lead to excessively pronounced irreversible sheet deformations, so-called clamping marks, in the region of the affected sheet corner of the printing material sheet.
As every one of the printing material sheets gripped one after another by the gripper system would have such clamping marks, problems would be expected as a consequence when stacking the printing material sheets on top of one another subsequently.
If there is provision for the printing material sheets to be stacked on top of one another so as to form a sheet stack by means of a sheet delivery of the machine comprising the gripper system, there would be an accumulation of the clamping marks. This is because the clamping mark of every printing material sheet would come to rest within the sheet stack exactly over the clamping mark of the printing material sheet lying underneath. A serious deviation of the sheet stack geometry from the required parallelepiped sheet stack geometry is to be expected in the region of the deformed (warped, rippled, etc.) sheet corners. This distortion of the sheet stack edges at least considerably impedes the further processing of the printing material sheets of the sheet stack or perhaps even makes the further processing completely impossible. This is because an exact sheet stack geometry is a precondition for the unproblematic further processing of the stack.